1. Field of the Invention
The invention relates to a device for treating liquids, in particular waste water, which comprises a surface aerator and also a mixer.
2. Prior Art
Aerators and mixers for treating waste water are known. By way of example, Belgian Patent BE 893,687 describes an aerator or mixer comprising a power source, attached to a float, which drives a shaft to which a screw pump is attached. In aerators of this type, the screw pump is usually surrounded by a water-guidance cylinder (pump housing), in which the water is pushed towards the surface of the water and is distributed over this surface. Below the screw pump, the pump housing may, for example, be shaped further to form a suction cone in order to promote the suction of water.
When treating waste water, it may be advantageous alternately to aerate the water (by introducing oxygen) and to mix the water (without the introduction of oxygen). The result is alternating processes of nitrification and denitrification which can be controlled as desired. In some cases, it may also be appropriate to aerate and mix simultaneously.
In deep pools, it is also possible to bring about simultaneous zones of aeration (nitrification) at the top and mixing (denitrification) at the bottom. If more than one unit is placed in a pool, it is possible to create different aeration zones and mixing zones which may be dimensioned and controlled in accordance with the requirements of the process.
Therefore it is not infrequently the case that a mixer is disposed fixedly in the pool, for example on the bottom of the pool or on a side wall, with the result that the two components are present as separate entities.
However, the fixed arrangement of a mixer in a pool is not an ideal solution, and such a mixer, if it is positioned on the bottom, is difficult to manipulate, to move or to maintain. For example, every manipulation requires additional material (bridge and hoisting crane) to be deployed. If the mixer is attached to the side wall of the pool, mixing is often limited to a specific zone and the desired mixing effect is not obtained. Especially in deep pools, it has been found that this arrangement does not allow an adequate mixing pattern to be achieved.
It has also already been proposed (for example in French Patent Application 87 00604) to lengthen the shaft of the aerator and to arrange a screw on this shaft in the pool, so that the appliance is simultaneously an aerator and a mixer.
However, such an apparatus has the drawback that the mixer and the aerator cannot function independently of one another and that, for example during a denitrification process, the aerator cannot be shut down. In other circumstances, it is not necessary for the mixer to function continuously together with the aerator in order, for example, to keep the particles in suspension. In many cases, it may be advantageous for the mixer and the aerator to be able to function alternately. Moreover, with regard to efficiency it is advantageous to save the power required to drive the mixer if mixing were to be unnecessary or disadvantageous for treating the water.
It is to be noted that U.S. Pat. No. 3,865,909 describes an aerating device having a submersible motor supported by a float member and a propeller which propels the water upwardly against a diffuser. This apparatus can be arranged to carry a second motor and propeller to move the water upwardly from deeper depths and thus to improve the action of the main submersible motor. However, this device does not create different aeration and mixing zones and both propellers do not function independently of one another to allow the creation of either a nitrification and/or a denitrification process. Furthermore, this device does not constitute an efficient and economic aeration process as the power source is situated under water and in the absence of a guidance housing can not create a strong upward movement of the water.